1. Field of the Invention
The present invention generally relates to a heating device and a magnetic recording head for thermally-assisted recording, and more particularly, a heating device and magnetic recording head which may be used for large-area thermally-assisted recording.
2. Description of the Related Art
In longitudinal magnetic disk recording, scaling to higher areal densities has historically relied on a continuous reduction of the magnetic grain size and the width of the grain size distribution in the recording media in order to keep media noise within acceptable limits. In contrast, the number of magnetic grains per bit cell has been reduced only slowly (e.g., see M. F. Doerner, K. Tang, T. Arnoldussen, H. Zeng, M. F. Toney, and D. Weller, “Microstructure and thermal stability of advanced longitudinal media”, IEEE Trans. Mag. 36, 43 (2000)).
The reduction of the average grain size has in turn led to a reduction of the stability factor for thermal magnetization reversal, which is given by the ratio of the stored magnetic energy KuV (where KU is the magnetocrystalline anisotropy, and V is the magnetic switching volume), to the thermal energy, kBT (wherein kB is the Boltzmann constant, and T is the absolute temperature). A minimum stability factor of about 65 is needed to avoid thermally driven demagnetization of bit transitions and, therefore, loss of data within the desired storage period of about 10 years (e.g., see D. Weller and A. Moser, “Thermal effect limits in ultrahigh-density magnetic recording”. IEEE Trans. Mag. 35, 4423 (1999)). Accordingly, grain size reductions can be compensated for by increases in KU and, therefore, the coercivity, HC, of the recording media.
However, while potential media materials with sufficiently high KU are known, this approach is limited to values of Hc lower than about half the maximum head write field in order for the head to be able to write to the media (e.g., see D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, M. F. Toney, J.-U. Thiele, and M. F. Doerner, “High KU materials approach to 100 Gbits/in2, IEEE Trans. Mag. 36, 10 (2000)). For the best write pole materials known today (e. g. CoNiFe alloys with a saturation field of about 2.4 T), this results in a write field of about 1.2 T for longitudinal recording, allowing a maximum Hc of about 6000 to 8000 Oe (e.g., see K. Ohashi, Y. Yasue, M. Saito, K. Yamada, T. Osaka, M. Takai, and K. Hayashi, “Newly developed inductive write head with electroplated CoNiFe film”, IEEE Trans. Mag. 34, 1462 (1998)).
Perpendicular magnetic recording using a single pole head and magnetic media with a soft magnetic underlayer is being investigated as a means to increase the effective head field by about a factor of 2 (e.g., see D. A. Thompson and J. S. Best, “The future of magnetic data storage technology”, IBM J. Res. Develop. 44, 311(2000)). Beyond this, a number of alternative solutions to extend magnetic recording towards areal densities in the range of 1 Tbit/in2 have been proposed.
Thermally-assisted magnetic recording aims to enable the use of media materials which have very high Ku (and are, therefore, stable against thermal magnetization reversal at reduced grain size) by temporarily heating the magnetic media and thereby lowering the coercivity of the media, during the magnetic write process. A number of experimental studies on thermally-assisted magnetic recording (TAR) have recently been published, mostly using a laser spot of 0.7-1 μm as a heat source to illuminate the back side of a single-sided magnetic recording medium, and writing with a conventional magnetic recording head (e.g., M. Alex, A. Tselikov, T. McDaniel, N. Deeman, T. Valet, D. Chen, “Characteristics of thermally-assisted recording”, IEEE Trans. Mag. 37, 1244 (2001)). Ultimately, however, any approach using a heat spot significantly larger than the track width will be limited by thermal erasure due to repeated heat exposure of adjacent data tracks during writing (e.g., see J. J. Ruigrok, R. Coehoorn, S. R. Cumpson, and H. W. Kesteren, “Disk recording beyond 100 Gb/in.2: Hybrid recording?”, J. Appl. Phys. 87, 5398 (2000)).
Based on simple geometry arguments, a maximum increase in areal density of about a factor of 2 for TAR using a large heat spot over “conventional” (i.e., without a heat spot) longitudinal magnetic recording can be estimated. Consequently, ultrahigh recording densities will require a sufficiently small and intense heat source such as, for example, very small apertures, solid-immersion lenses, or antennas fabricated directly onto the emitting surfaces of a laser diode or an optical waveguide. Earlier realizations of such systems suffered from insufficient power efficiencies and low data rates for writing and reading in all-optical systems (e.g., see E. Betzig, J. K. Trautman, T. D. Harris, R. Wolfe, E. M. Gregory, P. L. Finn, M. H. Kryder, C.-H. Chang, “Near-field magneto-optics and high density data storage”, Appl. Phys. Lett. 61, 142A (1992); and B. D. Terris, H. J. Mamin, D. Rugar, W. R. Studenmund, and G. S. Kino, “Near-field optical data storage using a solid immersion lens”, Appl. Phys. Lett. 65, 388 (1994)).
However, in a recent study laser diodes with sub-wavelength apertures (Very Small Aperture Laser—VSAL) were successfully used for optical recording on phase change media with mark diameters down to below 100 nm, raising the prospect of a TAR system for ultrahigh densities at reasonable data rates (e.g., see A. Partovi, D. Peale, M. Wuttig, C. A. Murray, G. Zydzik, L. Hopkins, K. Baldwin, W. S. Hobson, J. Wynn, J. Lopata, L. Dhar, R. Chichester, H. J. Yeh, “High-power laser light source for near-field optics and its application to high-density optical data storage”, Appl. Phys. Lett. 75, 1515 (1999)).
As it will be explained in more detail below, there are basically two different strategies to implement thermally-assisted recording, which we will refer in the following as large-area TAR and small-area TAR.
In small-area TAR, highly localized heating is realized (either by a local heater (e.g., see “High density magnetic thermal recording and reproducing assembly”, U.S. Pat. No. 6,233,206) or a near-field optical spot (e.g., see A. Partovi, D. Peale, M. Wuttig, C. A. Murray, G. Zydzik, L. Hopkins, K. Baldwin, W. S. Hobson, J. Wynn, J. Lopata, L. Dhar, R. Chichester, H.-J. Yeh, “High-power laser light source for near-field optics and its application to high-density optical data storage”, Appl. Phys. Lett. 75, 1515 (1999)), where the heat spot size is on the order of the bit dimensions. However, in practice, it turns out that small-area TAR faces several major challenges, especially if a heater in the recording head is used to realize the heating. Specifically, at a given power flux a small-area heat spot will result in significantly less heating in the disk than a large-area heat spot.
The situation is significantly improved in large-area TAR, where the heat spot exceeds the bit dimensions (e.g., realized by a large-area heater in a recording head or far-field laser heating analogous to magneto-optical recording (e.g, see M. Mansuripur, The physical principles of magneto-optical recording, Cambridge University Press, New York, 1995, p. 350). Although adjacent tracks will be heated in large-area TAR it can be shown using simple considerations, which are explained in more detail below, that moderate improvements in areal density can be obtained over the areal density obtained with small-area TAR.
For these reasons, it is advantageous to have a heating element in a recording head slider which is able to transfer energy to the media to assist in high areal density recording.